The Hall process of smelting aluminum uses electrolysis of alumina (Al.sub.2 O.sub.3) in a molten bath of cryolite (Na.sub.3 AlF.sub.6). The process initially starts with a powdered mixture of alumina and cryolite present in an open vessel or cell having steel walls lined with carbon, the cell being heated then by electric heaters to render the mixture molten at approximately 950.degree.-1000.degree. C. Carbon anodes are spaced from the carbon vessel walls (operating as the cathode) and a DC potential is applied between the anodes and the vessel walls producing the electrolysis current.
During the smelting cycle, the alumina breaks down to produce metallic aluminum (Al) which sinks in the molten electrolyte mixture to the bottom of the vessel. The normal starting concentration of the alumina in the electrolyte is ideally between 6 and 7 wt.%, and this is slowly reduced over an extended cycle usually in excess of 10-12 hours to approximately 2 wt.% or less. However, at some low level of alumina concentration (between perhaps 2.2 and 0.5 wt.% but more frequently between 2 and 1 wt.%) the alumina concentration is too low to maintain normal smelting, thereby precipitating what is known as the "anode effect".
Several adverse things happen during the anode effect. A coating of carbon tetrafluoride (CF.sub.4) builds up on the anode, preventing the bath from wetting the anode, and the voltage applied across the cathode and anode increases dramatically (from 4-6 volts to perhaps 20-40 volts or higher). This consequently brings about the second adverse factor, namely that because the current is kept constant (at approximately 600-800 milliamps per cm.sup.2 of anode), the power input goes up, the current efficiency drops, and the cell overheats. In overheating, the cell life is reduced, and extra chemicals in the form of fluoride (F) are consumed. The cycle is normally terminated after the anode effect phase of the cycle has already started.
To restart the cycle and remove the cycle from the anode effect, a solid crust formed over the top of the molten electrolyte in the cell is first broken and more powdered alumina is added, typically from a storage hopper mounted over the cell.
The metallic aluminum is siphoned from the bottom of the cell periodically, such as after several batches of alumina have been processed.
No reliable cycle control is known and/or commercially available that can measure alumina concentration. The corrosive and high temperature environment of the molten electrolyte makes most internal controls impractical. The most common control merely detects voltage changes between the cathode and anode, but this basically results only in an after-the-fact detection, as the anode effect has already started. The normal smelting cycle will operate in the 95-85% range of current efficiency during the major part of the cycle starting with the alumina concentrations in the 6-7 wt.% range to the low range where the anode effect begins; but thereafter in only a matter of minutes the current efficiency can drop dramatically to much less than 70 or even 50% current efficiency during the anode effect phase of the cycle. See FIG. 5 for a representative curve showing this relationship. The presently used cycle controls generally prove wanting as they lack sensitivity and fail to give advance warning.
The inaccurate control of the smelting cycle and the resulting "anode effect" that normally occurs every cycle waste tremendous amounts of energy and chemical efficiency, Several anodes generally are arranged side-by-side in a single cell perhaps 5 ft. high, 50 to 150 cells are typically connected together to form a single pot line, and perhaps 15-25 pot lines may be operating in a single commercial aluminum plant. A single cell can produce about 2,000 pounds of aluminum per day, but uses approximately 6 kilowatt-hours of electrical energy and 0.5 of a pound of carbon for every pound of aluminum produced. Considering U.S. aluminum smelting only, perhaps 6.times.10.sup.10 kilowatt hours/year could be saved with more accurate process control by automatically rebatching the process at alumina concentration of 2.5 or 2.7 wt.%, or above where the anode effect would normally ever first start.